FIELD OF THE INVENTION
The invention relates to an improved class of protective coatings for superalloy structural parts, especially for gas turbine vanes and blades.
In the field of gas turbine engines, designers continually look toward raising the operating temperature of the engine to increase efficiency. In turn, the oxidation rate of materials increases dramatically with increasing temperature. Gas turbine components can also be subjected to hot corrosion, when corrosive species are ingested into the engine via intake air and/or impurities in the fuel. Modern structural superalloys are designed for the ultimate in mechanical properties thereby sacrificing oxidation, and, to an even larger extent corrosion resistance.
To increase the useful life of gas turbine components it is customary to use protective coatings, such as aluminide or MCrAlY coatings where M may be Ni, Co, Fe or mixtures thereof. Since a coated turbine blade undergoes complicated stress states during operation, i.e. during heating and cooling cycles, advanced high temperature coatings must not only provide environmental protection but must also have specifically tailored physical and mechanical properties.
If the protective coating is to be used as a bond coat for thermal barrier coatings (TBCs) there are additional requirements. For an overlay coating, i.e. no TBC, the thermally grown oxide can spall and regrow provided that the activity of Al in the coating remains sufficiently high. For a TBC bond coat, oxide growth rate and oxide scale adherence are the life controlling parameters since if the oxide spalls, the TBC will spall. In summary, advanced high temperature protective coatings must have: a high oxidation resistance; a slowly growing oxide scale (low kp value); a good oxide scale adherence; a hot corrosion resistance, superior to SX/DS superalloys; a low interdiffusion of Al and Cr into the substrate to prevent the precipitation of brittle needle-like phases under the coating; a creep resistance comparable to conventional superalloys; a high ductility at low temperatures and a low ductile brittle transition temperature; and a thermal expansion coefficient similar to the substrate over the entire temperature range.
U.S. Pat. Nos. 5,273,712 and 5,154,885 disclose coatings with significant additions of Re which simultaneously improve the creep and oxidation resistance at high temperatures. However, the combination of Re with high Cr levels, which is typical with traditional coatings, results in an undesirable chase structure of the coating and the interdiffusion layer. At intermediate temperatures (below 950-900.degree. C.), .alpha.-Cr phase is more stable in the coating than the .gamma.-matrix. This results in low toughness and low ductility. In addition, a significant excess of Cr in the coating compared to the substrate results in diffusion of Cr to the base alloy, which enhances precipitation of needle-like Cr--, W-- and Rerich phases.
U.S. Pat. No. 4,758,480 discloses a class of protective coatings whose compositions are based on the compositions of the underlying substrate. The similarities in microstructure (gamma prime phase in gamma matrix) render the mechanical properties of the coating similar to the mechanical properties of the substrate, thereby reducing thermomechanically induced damage during service. However, the amount of Al (7.5-11 wt %) and Cr (9-16 wt %) in the coating may not provide sufficient oxidation and/or corrosion resistance for the long exposure times that are customary in stationary gas turbines.